1. Field of the Invention
The present invention relates to a liquid crystal display device using a vertical alignment mode which has excellent image display quality.
2. Description of the Related Art
In recent years, a liquid crystal display device is in widespread use as a display of a household electrical appliance such as a personal computer and a television set. In the liquid crystal display device, a liquid crystal panel is used which includes two glass substrates between which a liquid crystal is filled in a space surrounded by a sealing member.
For the liquid crystal display device as described above, a liquid crystal display device using a vertical alignment mode is commercially practical, in which a liquid crystal layer possessing negative dielectric anisotropy is interposed between an opposed glass substrate pair such that liquid crystal molecules are aligned vertically, and when a voltage is applied to the liquid crystal layer, the liquid crystal molecules are controlled to be aligned in a plurality of inclined directions relative to the glass substrates.
In this liquid crystal display device using the vertical alignment mode, the liquid crystal molecules are aligned vertically relative to the opposed glass substrate pair in a state where no voltage is applied thereto which defines a non-driven state, so that light which passes through the liquid crystal layer has its polarization plane little changed. Hence, by providing and arranging polarizing plates in the position of crossed Nicols on and under the glass substrates, black display can be achieved almost perfectly in the state where no voltage is applied thereto, and thereby a high-contrast image is obtained.
There arises a problem of viewing angle dependency in this liquid crystal display device using the vertical alignment mode, a similar problem arising also in a conventional liquid crystal display device using a TN mode; however, solutions to the problem are found as follows. Since, in order to improve viewing angle characteristics in display, it is effective to create a plurality of domains in one pixel for controlling liquid crystal molecules aligned in a vertical direction to be aligned in a plurality of inclined directions that are different from one domain to another relative to the vertical direction, protrusions are provided on an alignment layer or openings are provided in an electrode in the liquid crystal display device in order to create these domains.
As shown in FIG. 6, vertical-alignment-type liquid crystal molecules 53 which possess negative dielectric anisotropy tend to get vertical to an electric field direction 54 when a voltage is applied between electrodes 51 and 52. In this case, azimuthal directions of the liquid crystal molecules 53 become at random as shown in FIG. 6 if there is nothing to control the azimuthal directions. In contrast, the azimuthal directions are determined if there provided protrusions 55 and openings 57 as shown in FIG. 7. In this case, when a voltage is applied, the liquid crystal molecules 53 in the vicinities of the protrusions 55 and the openings 57 first start to incline before the liquid crystal molecules 53 in the other regions start to incline, and then the liquid crystal molecules 53 in the other regions start to tilt as if propagating through the regions, whereby the azimuthal directions of the aligned liquid crystal molecules 53 are controlled. Besides, in FIG. 7, a reference numeral 58 indicates equipotential lines at the time when a voltage is applied, and a reference numeral 59 indicates an oblique electric field (a fringe field).
Next, a description of a specific example of use of the above-described alignment control mechanisms will be given. As shown in FIG. 8, on an array substrate, a pair of gate bus lines 11 and a pair of source bus lines 12 which are perpendicular to each other are arranged in a grid pattern, and a pixel electrode 51 is provided in a pixel region surrounded by the gate bus lines 11 and the source bus lines 12. In the pixel electrode 51, openings 57 of slits are formed to extend in an oblique direction. The openings 57 are provided and arranged to generate an oblique electric field (a fringe field) at the time when a voltage is applied and to control liquid crystal molecule alignment in order to improve viewing angle characteristics as mentioned above. The openings 57 which are oblique at a given angle are arranged to be vertically symmetrical in each pixel electrode 51 as shown in FIG. 8.
Under the common electrode 52 (not shown in FIG. 8), linear protrusions 55a to 55c are provided. The linear protrusions 55a to 55c which are oblique at a given angle are arranged to be vertically symmetrical in each pixel electrode 51 in FIG. 8, and are placed at substantially center positions between the adjacent openings 57. The linear protrusions 55a to 55c are provided and arranged to align liquid crystal molecules in given inclined directions relative to the vertical direction in order to improve viewing angle characteristics.
The widths of liquid crystal domains which are defined by the openings 57 and the linear protrusions 55a to 55c are set to be optimum considering a vertical-alignment-type liquid crystal layer to be used, a voltage to be applied at the time of the lowest tone or a voltage to be applied at the time of the highest tone (see Japanese Patent Application Unexamined Publication No. 2002-229038).
The pixel electrode 51 having a substantially rectangular shape has four corner portions 51b, 51c, 51d and 51e, and among them, the upper left corner portion 51b, the upper right corner portion 51d and the lower right corner portion 51e where a TFT 13 is not located are rounded. The rounding is made in order to adjust the areas of the corner portions 51b, 51d and 51e to the area of the lower left corner portion 51c with a notch where the TFT 13 is located and adjust parasitic capacitances of the pixel electrode 51 and the bus lines 11 and 12. However, as a result of setting the widths of the liquid crystal domains defined by the openings 57 and the linear protrusions 55a to 55c to be optimum, the upper right linear protrusion 55c and the lower right liner protrusion 55c could be placed outside the corner portions 51d and 51e respectively.
FIG. 9 is a cross-sectional view along the line C-C in FIG. 8. In this case, the position where the equipotential lines 58 at the edge of the corner portion 51e of the pixel electrode 51 fall down, in other words, the position where the alignment of the liquid crystal molecules 53 is controlled by the oblique electric field 59 which is generated at the edge of the corner portion 51e, is located inside the position where the alignment of the liquid crystal molecules 53 is controlled by the linear protrusion 55c. Due to this, the azimuthal directions of the liquid crystal molecules 53 are not determined in this range, resulting in poor alignment of the liquid crystal molecules 53. This kind of poor alignment is visually perceived as irregular luminance on a liquid crystal display screen, which becomes a cause of loss of display quality.